In telephone switching systems such as private branch exchanges (PBX's) and key telephone systems a small number of telephone lines interconnect a central office with a larger number of telephone stations in the system. For example, a PBX may have 30 lines servicing 100 telephone stations while a key system may have 2 lines servicing 12 stations. Each station in the system has associated with it a unique line relay per telephone line for connecting that station to the particular telephone line. In the 2 line 12 station key system mentioned above, each of the 12 station sets would have 2 line relays for switching each station to either of the two lines, for a total of 24 line relays in the system.
Since each system has numerous line relays it is expedient that relatively low cost relays be used in order to minimize system costs and to achieve that result, low cost reed relays are preferred as line relays. However, dry reed relays can experience contact damage from power surges appearing on the telephone line when the station set is connected or disconnected from the line, thus decreasing the life of the relays and adding to maintainence and repair costs for the system.
There is a need, therefore, for a circuit arrangement to protect the low cost line relays from surges appearing on the telephone line without adding additional costs to the telephone system.